Date post: | 30-Mar-2015 |
Category: |
Documents |
Upload: | ansley-shield |
View: | 223 times |
Download: | 0 times |
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Innovative Processing for GaN Power Devices
Ilan Ben-Yaacov, Yan Gao, Sarah E. Monteith, S. DenBaars, U. Mishra, E.L. Hu
University of California, Santa Barbara
with thanks to
Andrew Huntington, Stacia Keller, Andreas Stonas (UCSB)
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Outline
• Next steps for (Al)GaAs-GaN HBTs– Wafer fusion
• Current Aperture Vertical Electron Transistor (CAVET)– Through MOCVD regrowth
– Through selective etching
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Structure• AlGaAs-GaAs emitter-base
– high mobility carriers– well-understood emitter-base interface– p contacts to GaAs base (rather than top GaN)
• n-GaN collector– High-breakdown voltages possible
n-GaN Collector
n-AlGaAs Emitter
p-GaAs Basefused
interface
In previous reviews• demonstrated reliable fusion of GaAs-GaN : 500 -750 oC, 0.5-2 hours• used SIMS, TEM, I-V measurements to characterize fused interface• Carried out initial electrical characterization of (Al)GaAs-GaN HBT
Formation of (Al)GaAs-GaN HBT
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
5 nm
Fused at 750oC for 15 minutes
Ex situ fusion and the fused interface
Spray etch to removeGaAs substrate
StartingMaterials
GaN
GaAs
Fuse under 2MPa pressureat 500-750°C for 0.25-2hr
GaAs
A uniform, relatively smooth interface
GaAs
GaN
Courtesy J. Jasinski
1 to 4monolayers
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
>2 m uid-GaN (~5x1016 Si)
(001) sapphire substrate
(1x1019 C)
120 nm n-AlxGa1-xAs(5x1017 Si, x = 0.3)
30 nm Graded AlxGa1-xAs(5x1017 Si, x = 0 - 0.3)
30 nm Graded AlxGa1-xAs(5x1017 Si, x = 0.3 – 0)
100 nm n-GaAs (1x1019 Si)
150 nm p-GaAs
Au/Ge/Ni415oC
Zn/Au
Al/Au
(Al)GaAs-GaN HBT Structure
GaAs/GaN Interfacefused at 750oC for 1 hour
Common Emitter Characteristic IB Step Size = 2mA
00.20.40.60.8
11.21.41.61.8
0 5 10 15 20 25 30
VEC (Volts)
Co
llec
tor
Cu
rren
t (m
A)
20 micron x 52 micron emitter mesa
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
(Al)GaAs-GaN HBT IV Characteristics
Common Emitter Characteristic IB Step Size = 2mA
00.20.40.60.8
11.21.41.61.8
0 5 10 15 20 25 30
VEC (Volts)
Co
llec
tor
Cu
rren
t (m
A)
• Relatively small VCE offset
(~1 V): can be improved with anneal p-GaAs contacts
• Reasonably good output conductance (~ mA; few hundred A/cm2)
• Low current gain (< 1)– large base width (150 nm)
– dopant profile after fusion? (AlGaAs-GaAs and GaAs-GaN)
– recombination at the fused interface?
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Gummel Plot
Gummel PlotHBT Sample H10
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0 0.5 1 1.5 2
VEB (Volts)
Cur
rent
(m
A)
IB (mA) IC (mA)
Common Emitter CharacteristicHBT Sample H10
Small Step in Base Current [0.1mA]
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 2 4 6 8 10
VCE (Volts)
Col
lect
or C
urre
nt (
mA
)
IB = 0 IB = 0.1mA IB = 0.2mA IB = 0.3mA
IB = 0.4mA IB = 0.5mA IB = 0.6mA IB = 0.7mA
IB = 0.8mA IB = 0.9mA
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
(Al)GaAs-GaN HBT: Next Steps
-5
0
5
10
15
20
25
-2 -1.5 -1 -0.5 0 0.5 1 1.5
Applied Bias (Volts)
Cu
rren
t D
ensi
ty (
A/c
m2)
6.5x10-3 Wcm2
Base-Emitter Junction
(pGaAs-nAlGaAs)
• Set fused interface slightly into collector region
- n-AlGaAs/p-GaAs/n-GaAs/n-GaN
- Allows for uncertainties in GaAs-GaN band lineups
- Previous experience indicates that n-AlGaAs/p-GaAs/n-GaAs structure will go through fusion process intact
• Reduce base layer thickness• Carry out fusion at lower temperatures
- Minimize dopant diffusion across fused interface
Same characteristicsbefore and after fusion
-3.5-3
-2.5-2
-1.5-1
-0.50
0.51
1.5
0 100 200 300 400 500 600 700 800
Depth from Emitter Surface (nm)
Ban
d E
nerg
y (e
V)
E B C
n-AlGaAsp-GaAs/ n-GaAs
GaN
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
• Current Aperture Vertical Electron Transistor
• Current flows vertically from sources to drain• Electron flow through aperture modulated by the gate• High-field region below the gate instead of at the surface (as in a HEMT)
• Higher breakdown voltages• When optimized, reduction in DC-RF dispersion
Source Source
Drain
AlGaN
Gate
225 Å
.25 m
2 m
GaN:Fe (semi-insulating) GaN:Fe.4 m
UID GaN
GaN:Si (n -type)
Sapphire Substrate
2DEG
High-Field Region
IDS
Regrown Channel GaN CAVET
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
1. MOCVD growth of drain and insulating regions
2. Cl2 RIE etch of aperture
region
3. MOCVD maskless regrowth of aperture and source region, pattern device mesa, and deposit metal contacts
CAVET Process Flow
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
CAVET: Theoretical Models
Ideal device, pinch-offoccurs between gate andaperture.
1.
When aperture regionis too insulating, pinch-off occurs across theaperture and currentdoes not saturate due toDIBL.
2.
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
CAVET: DC Electrical Results
For this device:• Ws = 2*Wg = 200 m, Lap = 0.6 m, and
Lgo = 2 m
• IDSS = 430 mA/mm, extrinsic gm = 100 mS/mm,
Vp = - 4 V
• Parasitic leakage current observed at pinch-off
For all devices:• IDSS and leakage current at pinch-off
independent of Lap
• IDSS and pinch-off leakage current increase
when Lgo is decreased
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Etched-Aperture GaN CAVET
InxGa1-xN (600Å) x = 0.070.5 micron uid- GaN
InGaN
1.7 microns n- GaN
AlGaNAlGaNAl0.30Ga0.70N (220Å)
sapphire
sapphire
1000 W Hg/Xe lamp (~20 W/cm2)
Au wire
Sample
KOH:H2O
GaN filter
• Create an etched aperture • Use an etch process that rapidly
and selectively etches a sacrificial layer (InGaN) PhotoElectroChemical (PEC) Wet Etching
Pt coil
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Bandgap-Selective PEC Etching
GaN Filter Transmission Spectra
0
0.2
0.4
0.6
0.8
1
320 360 400 440 480
Wavelength (nm)
Tra
nsm
issi
on
n-GaN Filter
p-GaN Filter
Approximate absorption edge of InGaN
h
GaN GaNInGaN
GaN filter will select wavelengths that only excite carriers in InGaN
GaN
sapphire
GaNInGaN
SiO2
Ni/Au
After Etching3 min, 1000 W, 2.2 M KOHI bias = 40 mA
PROBLEM: roughness of undercut etchRESPONSE: Taguchi experiment to identify most critical etch parameters
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
0.55M
2.2M
8.8M
2.71V
1.24V
0.62V
2.71V
1.24V
1.24V 2.71V 0.62V
170W 400W 600W
top view of undercutMRS
PEC etching
50µm
0.62V
Etched area Unetched area
Illuminationpower
KOH concentration• Systematically varied
- KOH concentration- illumination power- bias applied to
sample• Evaluated
- lateral etch rate- smoothness of etch
front- roughness of
etched surface• Overwhelming dependence on KOH concentration: lower concentration produced smoother etched surface
TOP-DOWN VIEWS OF ETCHED STRUCTURES
Optimization of Etch Conditions
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Etched CAVET: future work
• Taguchi experiments helped to identify critical etch parameter: KOH concentration
• Samples etched at 0.001 M KOH, 1000W, no bias, showed smooth, well-controlled undercut
• Future work: fabricate full CAVET device, using optimized etch conditions
Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors
NC STATE UNIVERSITY UCSB
Summary
• Initial electrical characterization of first (Al)GaAs-GaN fused HBT– Try ‘setback’ of fused interface, lower fusion temperature,
thinner base region
• Initial CAVET device results for regrown structures– Optimize device structure and growth conditions
• Optimization of PEC etching for etched CAVET devices– Fabricate and characterize full CAVET device